International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 3

Number 1

March 2009

Study on Ships Safety Control System

X.-J. Han, X.-Y. Meng & Z.-W. Wang

Dalian Maritime University, Dalian, China

1 PREFACE

Although modern ships have been equipped with

advanced facilities such as integrated bridge system,

automatic navigation device, unmanned engine

room, etc, fatal ships accidents still occurred from

time to time. Safety has been the key issue which

has restricted the development of ships. According

to the analysis of ships accidents statistical data from

IMO organization, there are three main kinds of rea-

sons which have caused accidents: ① equipment

failure in ship itself ② external environment(

weather, oceanic condition, etc)③ human factor.

human factor ships fault

exterior factor others

Figure 1. Proportion of reasons in ships accident

The proportion of these three reasons to the

whole is shown in Fig. 1. From which it is obvious

that more than eighty percent of ships accidents are

caused by human factor. All the accidents caused by

human factor due to wrong operation or misopera-

tion when navigators have poor qualities, lack of

watch, have poor capability in dealing with emer-

gency, do not master marine traffic rules enough,

etc. Therefore, in order to ensure safety navigating,

eliminating the error of human factor is the most im-

portant issue we should deal with firstly. Except that

the quality of navigator should be improved, ships

safety control system should be set up, which would

supervise ship’s navigating condition, discover po-

tential safety hazard in advance, and estimate the va-

lidity of operating order sent by a navigator. Espe-

cially in emergency, the system should make

estimation rapidly, and then give some appropriate

prompt, alarm, or orders of speeding down or block-

ing operations, so that safety can be ensured and ac-

cident can be avoided. The safety control system is

studied in this paper. How to forecast ships accident

and precaution of wrong order from navigator is the

focal issue to be solved

ABSTRACT: The proportion of main reasons of ships accidents to the whole reasons is discussed in the pa-

per. Among these reasons, human factors are in the majority. So a method to prevent wrong orders sent by a

navigator is laid emphasis on. On the basis of this, a ships safety control system is studied. The construction

and control principle of the ships safety control system, as well as control strategy, implementation method

and key technology are elaborated in the paper.

2 BASIC THOUGHT OF SAFETY CONTROL

SYSTEM

The ship navigation system is comprised of ship,

navigation environment and navigation technology

which is shown in Fig. 2.

Ship: a moving carrier which includes entity at-

tribute of a ship (weight, size, draught, etc), running

condition (course, navigational speed, etc), and mo-

tion characteristics (stowage, stability, heel, trim,

swinging period, etc.).

Navigation environment: refers to exterior cir-

cumstance when ship moves. It contains water area

environment, natural environment and transportation

environment. Transportation environment covers ob-

stacles in the area of navigation (fixed objects and

floating objects), prescriptive lane and traffic rules,

such as avoidance regulations, marine transportation

safety laws, etc.

Navigation technology is referred to the technol-

ogy and technique of navigating according to the

moving condition of the ship.

Figure 2. Construction of ship navigation system

It is obvious that among three factors of naviga-

tion system, the factor of ship is basically unchange-

able, the factor of navigation environment constantly

changes. The changes of environment do not lie on

human beings. The factor of navigation technology

is the drive technology that a navigator adopts based

on the former two factors, which includes watching

continuous, collecting information, comprehensive

analysis, and adjusting according to changes. There-

by, in the course of navigating, navigation technolo-

gy is the most decisive factor. It has been proved

through the facts that most shipwrecks and collision

accidents are caused by wrong operation or misoper-

ation.

In recent years, with regard to the constituent of

navigation system, great efforts have been made to

improve safety of ship navigation. They mainly con-

tain:

1 Improvement of ship: The ship size has been be-

ing bigger and bigger. Simultaneity, automation

of ship has been being improved greatly. Func-

tions of automatic navigation, supervision and

control have been making perfect constantly. And

hence ship manoeuvrability has becoming flexi-

ble and convenient, such as automatic navigation,

location, turning, shifting, emergency shut-down,

reversing, etc.

2 Improvement of navigation environment: Naviga-

tion environment is involved in transportation en-

vironment, sea area environment and natural en-

vironment. In which making transportation

environment better is easy to be achieved. In re-

cent years, masses of works have been done in

scientific setting and management of lane, work-

ing out ship collision regulations, improving and

perfecting transportation rules on sea, etc.

3 Improvement of navigation technology: For the

sake of raising navigators’ level of manoeuvre,

IMO organization attaches high importance to

improving the quality of navigators. They regu-

lated WTC convention (compulsory) strictly for

conforming and examining the process of training

for navigators, promotion and going on duty, so

that navigation technology can be improved.

All the efforts have played a great role in improv-

ing ships safety, otherwise, if we want to solve the

problem of ships safety radically, a specific ships

safety control system should be set up to predict ac-

cident potential and access the validity of orders sent

by navigator so that human error can be eradicated

completely. It is possible because the development

of information technology, computer and network

technology, as well as expert intelligent control

technology, etc.

3 CONSTRUCTION OF THE SYSTEM

The core functions of the ships safety control system

are forecasting accident potential and evaluating the

correctness of each order sent by navigators. On the

basis of equipments on board, a safety information

network is established which contains a host com-

puter, a server, the interface of data and network,

control output, etc. The construction of the system is

shown in Fig.3.

Figure 3. Construction of ships safety control system

The server is used for information integration, all

data involved in ships is stored in it as a database

and knowledge base. The interface of data and net-

work is connected to sensors which are used to

measure data related to safety ships, such as running

parameters of main engine (velocity of a ship, etc.),

running parameters of steering engine, information

of ARPA, data of GPS, etc. Some data are trans-

ferred from sensors to host computer directly, oth-

erwise most information are from network. The host

computer is the nucleus of the system. All kinds of

running data related to ships are collected then

communicated to the host computer, and according

to relevant information in the database, running con-

ditions of the ship will be calculated in real time

based on a model. Provided a potential risk is pre-

dicted, corresponding control will be outputted. The

control contains three kinds of outputs: prompting,

alarming, and blocking the error operations that

could cause severe dangerous effect.

4 SAFE SPEED AND ACCIDENT PRECAUTION

Ship trajectory is determined by the course and ve-

locity of a ship, noted as:

),(

iii

SVl

∑

. Provided that

in the area of a ship trajectory at a certain time, en-

counter objects varies in accordance with

Where, V is the velocity of a ship, n is the number of

objects (ships, fixed objects and floating objects), p

is the density of objects in the specific area. The en-

counter rate of the ship with other objects is:

VVVV

/1/

∞∞∞

(1)

It is obvious that encounter rate is inverse propor-

tional to velocity of the ship. Although increasing

the speed could bring down the encounter rate, but

the captain should also keep the ship at a safe speed

in the trajectory if he wants to keep the ships safely.

The concept of safe speed is put forward in IMO’s

new rules in 1972. It is defined as the speed relative

to water that it can die away completely before the

ship arrives at the collision point from anywhere.

The concept of safe speed is used as the main foun-

dation in this paper for forecasting ships accident.

That is, if actual velocity exceeds the safe speed at

any moment in the navigation, accident potential

would exist. Based on the scanning information

from ARPA radar, the object which is closest to the

ship’s course is regarded as a reference point at each

moment, safe speed could be computed. The period

of computing can be set, computing every 1 minute

in normal navigation, or computing every 1/2 or 1/4

minute in the area of narrow waters or that the densi-

ty of navigation object is greater.

There are many kinds of methods to calculate

safe speed. In this paper one method is adopted,

which is:

rsr

SDD

22 −−

(2)

where,

is the distance from the ship to the closest

object,

is the safe distance which is not more

than 1/2 of the distance that the target is in sight,

is the time from the target in sight to that an order is

sent,

is the sliding distance from that an order is

sent to that the ship stops completely.

is related to many kinds of factors, such as

navigation velocity, braking force, etc. In order to

shorten calculating time, a curve of navigation ve-

locity which is corresponding to rev of propeller and

braking distance should be stored in the host com-

puter. The curve shows the relationship of ship’s

true speed V (cable length/min) and rev of propeller

according to the result of speed measurement every

year on the measurement line, which is shown in

Fig.4. Whenever calculation, firstly, the value of

DD −2/

(cable length/min) is got which is shown

as point A in Fig.4. From the point A, a straight line

is drawn parallel to x-axis, which crosses with stop

line (

SS +

) at the point B (

(cable length) is the

sliding distance from that the target is in sight to that

an order is sent). Then From the point B, a perpen-

dicular line to x-axis is drawn, which crosses with

)(

at the point C, the rev of propeller can be got-

ten as a result. Finally, from the point of C, a straight

line is drawn parallel to x-axis, which crosses with

y-axis at point D. The speed in point D is the safe

speed.

Figure 4. The schematic of computing the safe speed

5 STRATEGIES OF THE SYSTEM

Strategies of safety control system designed in the

paper puts stress on calculating safe speed, then pre-

dicting accident according to information in data-

base and knowledge base. The strategies can be di-

vided into several types as follows:

1 When the ship is navigating at sea or on the broad

surface of a water area, there are few targets. The

safe speed calculated based on the closest dis-

tance of objects (very far generally) will be great-

er than the actual velocity (even greater than the

top speed). Risk rate is very small.

2 When the safe speed calculated is close to the ac-

tual velocity, it means that it is near with the clos-

est object. Risk rate is becoming greater. Potential

encountering risk exists. At this time, the system

should estimate that whether courses of them

have the possibility of crossing. If crossing, the

system should prompt the navigators.

3 When the safe speed calculated is less than the

actual velocity, it means that the ship is very

close to the object, risk rate is much greater. The

ship is possible to collide with the object. An

alarming signal will be sent immediately and

slowing down or stopping the ship according to

the difference between the safe speed and the ac-

tual velocity.

4 For each order sent by the navigator (rudder or-

ders and engine orders), the system will search

for the new closest object in the new course im-

mediately and calculate the safe speed according

to the change of course and velocity after the or-

der is sent. If the safe speed calculated after the

order is much less than the speed of the order, it

interprets that danger will occur. The system will

block the order at once so that it can not be sent

to engine room.

Figure 5. Strategy of safety control

For estimating the validity of an order, the system

should not only base on the relationship of actual ve-

locity and safe speed after the order is sent, but also

base on the expert prior knowledge stored in data-

base and knowledge base. Especially for emergency,

it is important whether the order conforms to the

emergency operation rules and collision avoidance

rules. Consequently, it is one of the focal works to

build a perfect safety database and knowledge base.

6 CONCLUSIONS

The prediction and safety control of ships accident is

made a probe in this paper. The method of predict-

ing an accident according to safe speed and estimat-

ing an order in real time is introduced. The key tech-

nology is integrating rules of safety relevant

information and real-time data processing method.

For the reason that there are many complex factors

including in the system, many aspects have not been

involve in this paper, such as processing rules after

information integration, the reliability of sea scan-

ning information, the influence of sea visibility, the

real-time requirement of calculating speed, the es-

tablishment of expert judgment system, etc. But it is

believed that the system of ships safety control sys-

tem must play a great role in improving navigation

safety by our efforts.

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